The prior art is replete with various transmissions for agricultural tractors and the like. Multi-speed transmissions having countershafts are widely used in the power train of a tractor arrangement because a plurality of rotating clutch assemblies and associated gears can be positioned on parallel shafts to allow considerable flexibility in adapting them to different space requirements. Moreover, a substantial number of simple and/or similarly sized parts can be used for manufacturing economy.
Known transmissions offer multi-ratio gear arrangements capable of providing varios ground speeds in a working (3-8 m.p.h.) range. These transmissions are adequate for some operations. For many operations, however, the speeds are too widely spaced or stepped apart to permit the tractor to be operated at or near a maximum speed for a given load imposed thereon. Also, the operator, for some conditions, must frequently stop the vehicle to change the driving relationship between the input shaft and output shaft of the transmission.
Accordingly, what is needed in the art is a simple, yet rugged and compact, transmission capable of producing progressively increasing tractor speeds in multiple, equal step gear ratios ranging between about 1.75 miles per hour and about 19 miles per hour. To reduce operator fatigue, such a transmission should be capable of effecting a powershift between successive gear ratios. Such powershifts must be smooth yet responsive and impart relatively low inertias to various powershift clutches disposed throughout the transmission. Furthermore, the transmission must be able to pick-up a heavy load without damage to the powershift clutches.